The invention relates to a method for determining the thickness of a contaminating layer and/or the type of a (at least one) contaminating material on a surface in an optical system, in particular on a surface in an EUV lithography system, to an optical element for reflecting EUV radiation, and to an EUV lithography system.
Contaminating materials may deposit on the surfaces of optical or non-optical elements arranged in an optical system, which materials may grow over the course of time to form a contaminating layer. One example of such an optical system is an EUV lithography apparatus in which EUV radiation-reflecting optical elements and non-optical components are arranged in a vacuum environment in a residual gas atmosphere in which the occurrence of contaminating gaseous substances cannot be completely avoided. Other examples of optical systems in which a contaminating layer may form are inspection or metrology systems for optical elements, for example for wafers or for masks. Such contaminating layers may also form in optical systems which use radiation at other wavelengths as used radiation, for example radiation in the DUV wavelength range. One example of such an optical system is a DUV lithography apparatus.
Information about the thickness of such a contaminating layer and the type of the contaminating materials forming the contaminating layer may be advantageous for cleaning purposes: by way of example, a respective surface may be cleaned if the thickness of the contaminating layer exceeds a predefined value, and/or the type of cleaning, for example the reagent used for the cleaning, for example a cleaning gas, may be chosen suitably, if appropriate, depending on the type of the contaminating material.
WO 2014/114382 A1 has disclosed a method for determining the phase angle at a free interface of an optical element provided with an EUV radiation-reflecting multilayer coating and/or for determining the thickness of a contaminating layer formed on the multilayer coating. In the method, a photocurrent generated when the multilayer coating is irradiated with EUV radiation is measured and the thickness of the contaminating layer is determined on the basis of a predefined relationship between the measured photocurrent and the thickness of the contaminating layer.
WO 2005/091076 A2 describes a method for determining the thickness of a capping layer of a multilayer coating on the basis of a phase shift of a standing electromagnetic wave. The phase shift of the standing wave is likewise measured with the aid of a photocurrent.
Besides the methods described above, the thickness of a contaminating layer may, if appropriate, also be determined by means of a measurement based on ellipsometry. However, a defined geometry between the source of the measurement radiation and the detector is necessary in this case, such that there is very high complexity involved in performing a (scanning) measurement on an optical element in the form of a mirror or on a plurality of facets of a facet mirror and in determining the thickness of the contaminating layer in this way.